1. Field of the Invention
This invention relates to a thin film transistor and a semiconductor device and a method for forming the same, more particularly to a method of producing a silicon thin film transistor for use in an active matrix drive type liquid crystal display, an image sensor, a thermal head, etc., and to a silicon thin film transistor manufactured by the same.
2. Description of Prior Art
A silicon thin film transistor has been hitherto utilized in a liquid crystal display for a compact TV or computer, and an active element or a part of a drive circuit for an image sensor and a thermal head which are used in a facsimile machine, etc. An amorphous silicon thin film transistor is being most vigorously developed now because it can be manufactured relatively easily and designed in a large area.
The amorphous silicon thin film transistor, however, has a demerit that electron and hole mobilities are very small, 1 cm.sup.2 /Vsec and 0.1 cm.sup.2 /Vsec respectively. This is not a critical problem, for example when it is used to perform a switching operation of each pixel of a liquid crystal display or each bit of an image sensor, but is a critical problem particularly in a switching speed when a drive circuit is constructed the same on substrate.
On the other hand, a polycrystal silicon thin film transistor which has been used for a compact-size liquid crystal TV or an image sensor has electron and hole mobilities of about 10 cm.sup.2 /Vsec, and some driving circuits comprising polycrystal silicon thin film transistors are actually marketed as a product. However, the polycrystal silicon thin film transistor is also insufficient to satisfy scale-up and speed-up needs of a device. The polycrystal silicon thin film transistor has polycrystal silicon film formed in an active layer by an LPCVD method, for example. Such polycrystal silicon has a high potential barrier at a grain boundary, so that both of electron and hole mobilities are limited to about 10 cm.sup.2 /Vsec or less. In addition, in most cases, cost is high because of use of expensive quartz as a substrate.
Apart from such amorphous silicon thin film transistor or polycrystal silicon thin film transistor, another method in which amorphous silicon film is formed on a substrate formed of quartz or glass, and then heated for a long time to be grown in solid phase and crystallized has been recently studied. This method provides a thin film transistor having a high mobility of several times to several-tens times of that of a polycrystal silicon thin film transistor (this is dependent on a process condition), and also has high productivity, so that this method is more effective for the next generation's active matrix drive liquid crystal display having driving circuits formed on the same large substrate, for example.
Since a quartz substrate is durable to high temperature above 1000.degree. C., the following advantages are obtained: a gate insulation film formed by thermal oxidation of silicon active layer, which is used generally in silicon wafer process, can be adopted, and a quartz substrate provides a good silicon/silicon-oxide interface. However, the thin film transistor grown in solid phase on the quartz substrate by above method has not only a problem that stable and uniform characteristics cannot be obtained because of lack of uniformity of thermal contraction of quartz, but also a problem that there occurs a failure due to an instability phenomenon that reduction of off resistance and current-curve variation with time lapse are developed, and these problems make it difficult to realize large-area device. In addition, the quartz substrate is more expensive several times than glass, especially for a large-area substrate, so that the quartz would be unsuitable as substrate in this respect.
On the other hand, relatively-low cost glass which is durable to a thermal process of about 600.degree. C. at maximum has been recently developed. Some kinds of this glass can be used for a solid-phase thermal growth process below 600.degree. C. because they contain a small amount of alkali component. These kinds of glass show a very large coefficient of thermal contraction by heating them at 600.degree. C. for 24 hours, for example. Therefore, when a thin film transistor is formed on such a glass substrate, a threshold voltage is observed to be shifted to a positive side by 2 to 5 volt because the substrate is thermally contracted by a heat-treatment in a crystallization process of the semiconductor film, so that a compression stress acts on an active layer of the thin film transistor and the interface condition of the active layer silicon/gate insulator layer is deteriorated.
In addition, since the thermal contraction of the substrate exceeds 200 ppm in a heating process for activating impurities doped in the thin film transistor, a pattern alignment is difficult even using a reduction projecting light exposure device having a mechanism of correcting a substrate expansion and contraction, especially when the diagonal length of the substrate exceeds 10 inches.